FIG. 10A is an exploded perspective view showing a structure of a conventional wound electric double-layer capacitor used in various electronic devices. FIG. 10B is a developed perspective view showing a structure of a capacitor element used in the capacitor. In FIG. 10A and FIG. 10B, capacitor element 431 is formed by winding positive electrode 434 and negative electrode 435 in a state where separator 436 is interposed between them.
Each of positive electrode 434 and negative electrode 435 is formed by disposing polarized electrode layers (not shown) on both surfaces of a current collector (not shown) made of a metal foil, and positive electrode lead wire 432 is coupled to positive electrode 434, and negative electrode lead wire 433 is coupled to negative electrode 435.
Capacitor element 431 having such a structure is impregnated with an electrolyte for driving (not shown), and then inserted into closed-end cylindrical metal case 438. Rubber-made sealing member 437 having holes into which positive electrode lead wire 432 and negative electrode lead wire 433 are inserted is disposed on the opening of metal case 438. Then, the outer periphery of the opening of metal case 438 is drawing-processed to the inside to perform sealing.
The polarized electrode layers formed in positive electrode 434 and negative electrode 435 are produced by mixing carbon black as activated carbon powder and a conductive adding agent, polytetrafluoroethylene as a binder, a water-soluble binder of carboxymethyl cellulose (hereinafter referred to as “CMC”), sufficiently kneading them with a kneader to produce paste, applying this paste to front and rear surfaces of the current collector made of an aluminum foil, and drying them. Positive electrode 434 and negative electrode 435 are formed in substantially the same size.
For improving the coupling of positive electrode lead wire 432 and negative electrode lead wire 433 to the aluminum foil of the current collector, the coupling part is formed by partially removing the polarized electrode layers to expose the aluminum foil of the current collector.
Conventional art document information related to the present invention is disclosed by patent document 1 and patent document 2, for example.
FIG. 11A is a sectional view showing a structure of another conventional wound electric double-layer capacitor. FIG. 11B is a developed perspective view showing the structure of a capacitor element used in this capacitor. FIG. 11C is a perspective view of this capacitor element. In FIG. 11A, FIG. 11B, and FIG. 11C, capacitor element 531 is stored in metal case 538 together with metal plate 537 and electrolyte for driving (not shown), and the opening of metal case 538 is sealed by terminal plate 540 via rubber 539 for sealing, thereby forming the conventional wound electric double-layer capacitor.
Capacitor element 531 is described hereinafter. Capacitor element 531 is formed by stacking and winding band-like anode foil 532 and band-like cathode foil 533 through band-like separator 534. Anode foil 532 has drawing electrode 535 on one longitudinal side thereof, and cathode foil 533 has drawing electrode 536 on the other longitudinal side thereof.
Anode foil 532 and cathode foil 533 are described hereinafter. Anode foil 532 and cathode foil 533 are electrodes that have a polarized electrode layer on a metal foil (not shown) and are formed in substantially the same size. The polarized electrode layer is produced by mixing carbon black as activated carbon powder and a conductive adding agent, polytetrafluoroethylene as a binder, a water-soluble binder of carboxymethyl cellulose (CMC), sufficiently kneading them with a kneader to produce paste, applying this paste to the surface of the metal foil, and drying them.
Drawing electrodes 535 and 536 are welded to the inner bottom surface of metal case 538 and the inner surface of metal plate 537, respectively. Metal plate 538 and metal case 537 are coupled to outer terminals 541 and 542, respectively.
For improving the coupling between metal case 538 and metal plate 537, polarized electrode layers are formed on surfaces of anode foil 532 and cathode foil 533 except the ends of the metal foils. Alternatively, the ends having no polarized electrode layer may be used as electrodes 535 and 536 by removing the polarized electrode layers formed on the surfaces of the ends of the metal foils.
As a conventional example, the outermost periphery of capacitor element 531 is made of cathode foil 533. When polarized electrode layers are formed on both surfaces of the metal foil, however, separator 534 may be disposed on the outermost periphery so as to prevent the contact of metal case 538 with the polarized electrode layers.
Conventional art document information related to the present invention is disclosed by patent document 3, for example.
However, the former conventional wound electric double-layer capacitor is designed so that the capacity of positive electrode 434 is equal to that of negative electrode 435. Therefore, when voltage is applied to the electric double-layer capacitor, positive electrode 434 and negative electrode 435 are polarized from the spontaneous potential by the same width. This polarization causes electrochemical reaction on the surface of the polarized electrode layer of each electrode to cause characteristic degradation such as gas generation, resistance increase, and capacity reduction, disadvantageously.
When positive electrode 434 and negative electrode 435 are wound via separator 436, positive electrode 434 becomes slightly longer than negative electrode 435, and positive electrode 434 sometimes protrudes. When the wound electric double-layer capacitor is used for a long time in a state where positive electrode 434 protrudes, the electrolyte for driving causes chemical reaction between the tip of negative electrode 435 and positive electrode 434, BF4−, PF6− or the like of electrolyte anion is attracted to the positive electrode side, the periphery of the positive electrode becomes acidic to degrade separator 436, and the characteristic degrades, disadvantageously.
In this conventional wound electric double-layer capacitor, for improving the coupling of positive electrode lead wire 432 and negative electrode lead wire 433 to the aluminum foil of the current collector, the coupling part of each lead wire is coupled to the part of the aluminum foil that is exposed by partially removing the polarized electrode layer. Here, the polarized electrode layer is disposed on the other electrode side facing the lead wires through the separator. In the capacitor having such a structure, the potential difference between the coupling part of negative electrode lead wire 433 coupled to negative electrode 435 and positive electrode 434 facing it through separator 436 is larger than that between the other positive electrode 434 and negative electrode 435. Therefore, when this capacitor is used for a long time, the electrolyte for driving causes chemical reaction between them, BF4−, PF6− or the like of electrolyte anion is attracted to the positive electrode side, the periphery of the positive electrode becomes acidic to degrade separator 436, and the characteristic degrades, disadvantageously.
While, an alkaline component is attracted to the negative electrode side even between the coupling part of positive electrode lead wire 432 coupled to positive electrode 434 and negative electrode 435 facing it through separator 436, and the characteristic degrades disadvantageously though separator 436 does not degrade.
This conventional wound electric double-layer capacitor is designed so that the capacity of positive electrode 434 is equal to that of negative electrode 435. Therefore, when voltage is applied to the electric double-layer capacitor, positive electrode 434 and negative electrode 435 are polarized from the spontaneous potential by the same width. This polarization causes electrochemical reaction on the surface of the polarized electrode layer of each electrode to cause characteristic degradation such as gas generation, resistance increase, and capacity reduction, disadvantageously.
The latter conventional wound electric double-layer capacitor is designed so that the capacitance of anode foil 532 is equal to that of cathode foil 533 and the sizes and shapes of them are substantially the same. Therefore, when voltage is applied to the electric double-layer capacitor, anode foil 532 and cathode foil 533 are polarized from the spontaneous potential by the same potential, charge is generated on the surface of each electrode, electrochemical reaction occurs on the surface to cause characteristic degradation such as gas generation, resistance increase, and capacity reduction.
Especially when anode foil 532 and cathode foil 533 are wound via separator 534, anode foil 532 becomes longer than cathode foil 533. When the wound electric double-layer capacitor is used for a long time in a state where anode foil 532 protrudes at the winding end of capacitor element 531, the electrolyte for driving can cause chemical reaction between the winding end of cathode foil 533 and anode foil 532, BF4−, PF6− or the like of electrolyte anion is attracted to anode foil 532 side, the periphery of the anode foil becomes acidic to degrade separator 534.
[Patent document 1] Japanese Patent Unexamined Publication No. H10-270293
[Patent document 2] Japanese Patent Unexamined Publication No. H09-17695
[Patent document 3] Japanese Patent Unexamined Publication No. 2000-315632